This is not difficult, but you have to know the tricks. Since this site does not allow uploading of photos (a pity), you have to first store them somewhere where they are accessible online. I use imgur.com. Just open an account (it's free) and then follow the instructions they provide to upload your photos.

Once you do this, display the photo you wish to use on their site. Right-click the mouse on it and select "Copy link address". Then within this forum's edit box, click on the "Img" button on the top menu. It will display "{img}{/img}" (but with square brackets in instead of curly braces). Place the cursor right between the middle brackets and then right-click on the mouse and select "Paste".

It should look something like this: {img}https://imgur.com/gallery/maruLHY{/img}

Now click on the Preview button in the editor's bottom menu. It will show the photo you selected. If all is well then select Submit.